The invention relates to the field of vertically coupled optical resonator devices, and in particular to vertically coupled optical resonator devices arranged over a cross-grid waveguide architecture.
Ring and disk resonators fabricated on optical substrates have been investigated theoretically and experimentally for their potential use in optical signal processing applications. It is desirable for the ring or disk dimensions to be as small as possible, so that the free spectral range of the resonances is large. Typically, such micro-resonators are a few tens of the operating wavelengths in diameter, or smaller. In order for the optical signals to be guided around a tight radius of curvature, the dielectric index contrast between the ring waveguide and the surrounding medium in the plane of the ring must be large.
Waveguides with large core/cladding index contrasts are called tightly confined waveguides. Tightly confined ring resonators have been fabricated in the Si/SiO2 material system (see Little et. al. xe2x80x9cUltra-compact Si/SiO2 micro-ring resonator channel dropping filterxe2x80x9d, IEEE Photonic. Tech. Lett., vol. 10, pp. 549-551, 1998), and the GaAs/AIGaAs material system (see Rafizadeh et. al. xe2x80x9cWaveguide-coupled AlGaAs/GaAs microcavity ring and disk resonators with high finesse and 21.6 nm free spectral rangexe2x80x9d, Opt. Lett. vol. 22, pp. 1244-1246, 1997). Larger radius rings have also been fabricated in glass (K. Oda et. al. xe2x80x9cA wide-FSR waveguide double-ring resonator with a wide free spectral range of 100 GHz,xe2x80x9d IEEE J. Lightwave Technology, vol. 13, pp. 1766-1771, 1995). In all these structures, the rings and the waveguides that couple signals into the rings are all in the same planar layer.
FIG. 1a shows a top down view, while FIG. 1b shows a cross-sectional view, of a simplified schematic diagram of a typical planar microring resonator device 100. In this case, a ring 102 is side-coupled to a pair of bus waveguides 104, 106. This geometry is called a xe2x80x9claterally coupled ringxe2x80x9d. In laterally coupled microring resonators, the ring is separated from the bus waveguides by an ultra-thin etched gap 108. The resonator performance depends exponentially on the width of the gap, and is therefore affected significantly by fabrication deviations. In addition, because the ring and the bus waveguides arc in the same planar layer, they are both fabricated from the same materials and have similar geometries. However, it is desirable to fabricate the bus waveguides differently than those used for the rings, so that each can be independently optimized for its particular role in the overall device.
In the conventional vertically coupled arrangement, the bus waveguides are placed above the ring, and are separated from the ring by a buffer layer. The waveguides and the ring are also buried below the surface of the chip. FIG. 2 shows a cutaway view of a simplified schematic diagram of a ring resonator device 200 having a ring resonator 202 buried below a pair of vertically coupled waveguides 204, 206. Vertical coupling of the ring to the bus reduces the sensitivity to misalignments between the two.
Furthermore, etching of very narrow gaps 208 is not required. Conventional vertically coupled rings have been fabricated in glass (see Suzuki et. al., xe2x80x9cIntegrated-optic ring resonators with two stacked layers of silica waveguide on Sixe2x80x9d, IEEE Photonic Tech. Lett., vol. 4, pp. 1256-1257, 1992). Because the ring is buried, its core-to-cladding refractive index contrast is low. In order to avoid bending induced losses, such low-index contrast rings need to have a large radius. In addition, because the ring is buried it is not easily accessible for trimming or tuning.
Vertical-type coupling has been proposed for GaAs/AlGaAs type ring resonators (Chin et al., xe2x80x9cDesign and modeling of waveguide-coupled single-mode microring resonatorsxe2x80x9d, IEEE J. Lightwave Technology, vol. 16, pp. 1433-1446, 1998), and also fabricated by bond-and etch back methods (Tishinin et al., xe2x80x9cNovel fabrication process for vertical resonant coupler with precise coupling efficiency controlxe2x80x9d, in LEOS 11th annual meeting, Institute of Electrical and Electronics Engineers, paper TuK5, 1998). In these arrangements, the bus waveguides sit on narrow pedestals, are fixed by the positions of the rings, and have other problems associated with efficient coupling to optical fibers.
One of the many desirable attributes of microresonators discussed above is that they are much smaller than other optical devices, and so many more devices can be accommodated on a single chip. Complex optical circuits can be envisioned by interconnecting hundreds to thousands of resonator devices. All microresonator devices proposed to date use high index contrast bus waveguides. Such waveguides can not physically cross through one another without considerable scattering losses and cross-talk. For these reasons, general interconnect architectures are restricted to those having non-intersecting waveguides. Only one article has addressed interconnection architectures, and it focused exclusively on the non-intersecting waveguide types, (see Soref et al., xe2x80x9cProposed N-wavelength M-fiber WDM crossconnect switch using active microring resonatorsxe2x80x9d, Photonic Tech. Lett., vol. 10, pp. 1121-1123, 1998).
In accordance with the invention, the disadvantages of prior optical resonator devices and resonator device architectures have been overcome. The resonators of the invention are vertically coupled on top of the bus waveguides, and are separated from the waveguides by a buffer layer of arbitrary thickness. This vertical arrangement eliminates the need for etching fine gaps to separate the rings and guides. It reduces the alignment sensitivity between the desired position of the resonator and bus waveguides by a significant degree. The ring and bus waveguides lie in different vertical layers, and each can therefore be optimized independently.
For example, a ring resonator can be optimized for higher index contrast in the plane, small size, and low bending loss, while the bus waveguides are designed to have lower index contrast in the plane, low propagation losses, and dimensions that make them suitable for matching to optical fibers. The waveguides can also have any lateral placement underneath the rings and are not restricted by the placement of the rings. Furthermore, with the rings lying on the top layer of the structure, they are easily accessed for tuning and trimming. Unlike vertically coupled ring resonators of the prior art, in which the ring and bus waveguides were both buried, or both air-clad, the invention seeks to have bus waveguides that are buried and ring waveguides that are air-clad.
Unlike conventional microring resonator designs, the bus waveguides in the invention can have a low core/cladding index contrast in the plane. Because of these low index contrasts, bus waveguides can now physically cross through one another without causing significant scattering loss or cross-talk on the optical signals. Waveguides that cross are essential for designing arbitrary large-scale integrated optical circuits, with which the invention is also concerned. The invention provides a means and the details for systematically constructing large-scale architectures for multi-resonator devices.
In addition, a scalable architecture using vertically coupled microring resonators above crossing waveguides is provided. The architecture consists of waveguides that cross through each other on a Manhattan-like grid pattern. Near each waveguide crossing junction, a resonator device is vertically integrated. Each ring in the architecture can perform a different signal processing function, including that of an add/drop filter, ON/OFF switch, amplitude and phase modulator, dispersion compensation, polarization rotator, polarization splitter, optical tap, to name a few.
In accordance with one exemplary embodiment, the optical resonator device of the invention includes three or more distinct layers. A micro-cavity resonator, or collection of coupled resonators, is formed in one layer. In a second layer, there is one or more bus waveguides underneath the resonators. The bus and ring layers are separated by a third layer, called a buffer layer, which can have an arbitrary thickness. Each of the layers and the waveguides within those layers can be composed of a different dielectric material. The waveguides can have any orientation in the plane of the layer, and can cross through each other at arbitrary angles. These crossings allow the entire surface of the chip to be accessed without the need to bend the bus waveguides. It also allows arbitrary optical circuits to be realized. For example, by cascading N resonator devices, a 1xc3x97N add/drop filter is easily realized for wavelength division multiplexed optical communications. An Nxc3x97N array can be used for switching arrays.